Bistable deformable mirror devices (DMDs) are the subject of the above-listed copending U.S. patent application entitled SPATIAL LIGHT MODULATOR, U.S. Pat. No. 5,061,049. There are many uses for such devices, such as, for example, a projection light valve for high definition television (HDTV). In certain of these applications, it must be capable of supporting at least 128 grey levels. Because the bistable DMD is a binary device, grey levels are achieved by pulse-width modulation.
There are two approaches to pulse-width modulation. The first approach is to divide the video frame period .tau..sub.f into 128 equal time intervals or fields. During each field, all of the pixels in the array are addressed to one of two states; the positive landing angle (on) or the negative landing angle (off).
The second approach is to express the 128 equal time intervals as combinations of seven time intervals, each interval being a multiple of two times the previous interval. The seven time intervals would be the multiples 1, 2, 4, 8, 16, 32, and 64 of .tau..sub.f /128. This approach minimizes the amount of off-chip frame buffer memory that is required.
The shortest field time is .tau..sub.f 128, regardless of which pulse-width modulation approach is chosen. For a DMD projection light valve system where the three color fields are generated in parallel during the standard TV frame time of 33.3 mS, the shortest field time is .tau..sub.f /128=260 .mu.s. During this 260 .mu.s time interval, the 1050 lines of the DMD must be updated, one line at a time. If the bottom 525 lines were to be addressed in parallel with the top 525 lines, then the time available for each line update would be 260 .mu.s/525=0.5 .mu.s.
It is not currently possible to line-update the DMD within a time interval as short as 0.5 .mu.s, because a resonant reset must be applied to the deformable beam prior to each line update. This resonant reset consists of a pulse train typically 2 .mu.s long (.tau..sub.1) followed by a beam settling time of 12 .mu.s (.tau..sub.2) before the differential bias can be reapplied. Its purpose is to unstick the beam from its landed position in contact with the landing surface.
Accordingly, an alternate update scheme must be employed for DMDs running at TV rates and supporting 128 grey levels.